In a case where it is detected that a temperature of a fuel cell stack is below a freezing temperature (below a freezing point) of water when a fuel cell system is started, a control map for normal starting is changed to a control map for starting below the freezing point, and the fuel cell is started along the map for the starting below the freezing point. There is an art that startability is improved by performing, in this case, a rapid warm-up operation which rapidly raises a temperature at an electric power generation part to exceed the temperature below the freezing point.
The rapid warm-up operation (electric power generation at low efficiency) means electric power generation in which a volume of a reactant gas supplied to a fuel cell is smaller than that in normal electric power generation, and an electric power loss is larger than that in the normal electric power generation, to operate the fuel cell in a state of a reduced air stoichiometric ratio. The fuel cell can rapidly be warmed up by intentionally setting the electric power loss to be larger in this way. On the other hand, in the case of the normal electric power generation, the fuel cell is operated in a state where the air stoichiometric ratio is set to a predetermined value or higher so that high electric power generation efficiency can be obtained by suppressing the electric power loss.